This invention relates to monolithic integrated circuit structures. More particularly, it is concerned with the fabrication of complementary modulation-doped field effect transistors.
Interest in high-speed digital circuitry and high frequency microwave devices has prompted investigation of the gallium arsenide/aluminum gallium arsenide (GaAs/AlGaAs) material system because of its inherent speed advantages over silicon. One type of device which has been developed is the modulation-doped field effect transistor (MODFET). The structure of this device includes an arrangement of layers of GaAs and AlGaAs. A thin potential well is formed in a layer of undoped GaAs which is adjacent to and may be spaced from a layer of heavily doped N-type AlGaAs. Electrons from the heavily doped AlGaAs fall into this potential well forming a two-dimensional electron gas (2DEG) in which electrons have high mobility due to the absence of ionized impurity scattering in the undoped GaAs. A Schottky barrier gate on the surface of the N-type AlGaAs is employed to control the conduction of the two-dimensional electron gas between source and drain regions formed in the undoped GaAs. This device operates in a manner similar to a silicon N-type MOS transistor, except that the high mobility of the two-dimensional electron gas in the undoped GaAs results in a much larger transconductance and thus a higher frequency of operation than are obtainable with the silicon device.
Similar devices which employ an acceptor layer of P-type AlGaAs and thus utilize a two-dimensional hole gas (2DHG) have recently been developed. This device operates in a manner similar to a silicon P-type MOS transistor. The use of complementary pairs of silicon MOS transistors (CMOS) to provide low power dissipation circuitry is well known. Similarly, it would be desirable to fabricate integrated circuit structures incorporating complementary pairs of modulation-doped field effect transistors. The fabrication of such devices, however, employing conventional processing procedures used in the fabrication of the two types of devices individually would be complex and expensive.